1. Field of the Invention
The present invention relates to flattening method and apparatus for semiconductor device, and particularly to a flattening method and apparatus with the improved precision of detecting (or determining) a polishing end point for polishing the surface of semiconductor device.
2. Description of the Related Art
Recently, as the elements in the semiconductor device have been highly integrated and fine patterned, the multilayering technique such as polycrystalline silicon wiring has been progressed to realize faster operation and higher electric performance of the semiconductor device. In the manufacturing process for semiconductor device, when an irregular portion, or uneven level is produced on the surface of the silicon wafer substrate with a lower wiring conductor formed thereon (which is hereinafter referred to as "device wafer"), an upper wiring conductor formed on the surface of the device wafer may cause a short circuit between the conductors or the control of focusing on the surface of the device wafer upon exposure may be difficult in a fine patterning operation such as photolithography process.
In the prior art, the surface of the device wafer is flattened by CMP (Chemical Mechanical Polishing). However, in order to assure a suitable degree of polishing or correct depth of polishing, it is necessary to precisely determine a polishing end point which means the time or timing when the polishing process ends.
A polishing end point detector disclosed in JP-A-6-315850 measures the torque of at least one of the first shaft for driving the polishing table, on which an object to be polished such as a silicon wafer is placed, and the second shaft for rotating the polished object, and then detects that at least one of the measured torque and the differentiated value of the measured torque with respect to time has changed to a predetermined value or above, thus producing a polishing end signal which determines the polishing end point.
In this conventional polishing end deciding method based on the torque change of the shaft, the information for the decision is obtained from the torque change over the entire wafer, and thus the decision is not always exact. In other words, it is uncertain if a torque change in the shaft is caused by part of the wafer or by the entire wafer, and hence such decision is unavoidably macroscopic and rough.
Other conventional polishing methods as given below are disclosed which employ a strain gauge helpful for precisely polishing the surface of semiconductor device.
(1) Semiconductor substrate polishing control method in JP-A-2-257630
During polishing, the amount of deformation of the lower surface of a holder which supports and presses a semiconductor substrate thereon is measured by a strain gauge, and the upper and lower portions of the holder are held at different temperatures in proportion to the measured amount of deformation, thereby regulating the surface shape of the lower surface of the holder. In addition, during polishing, the amount of deformation of the polishing cloth (or the abrasive cloth) is measured by a displacement meter, and the temperature of the polishing liquid (or the abrasive liquid) poured on the polishing cloth is determined in accordance with the measured amount of deformation, thereby regulating the surface shape of the polishing cloth. Moreover, the amount of deformation of the lower table with the polishing cloth attached on the surface is measured by the displacement meter, and the upper and lower portions of the lower table are held at different temperatures according to the measured amount of deformation, thereby regulating the surface shape of the lower table.
(2) Heat deformation control type surface polishing method in JP-A-4-53671
A plurality of heaters and a plurality of strain gauges are provided on the opposite surface to the polishing side of the upper table, and the heaters are turned on and off in accordance with the amount of deformation of the upper table measured by the strain gauges during polishing, thereby stabilizing the deformation of the upper table due to heat.
However, none of the two conventional methods given above improves the precision in determining the polishing end point by use of strain gauges.